Natural gas, however produced, may contain acid gas impurities, particularly carbon dioxide and hydrogen sulfide. Such natural gas is referred to as "sour gas," meaning that the "sweet" natural gas is contaminated with an unacceptable quantity of "sour" acid gas impurities. The presence in natural gas of such acid gas impurities is objectionable, among other reasons, because of the corrosion problems they cause during pipeline transmission of the natural gas to market and because acid gas contaminants reduce the BTU content per standard cubic foot of the natural gas product. Natural gas produced from a field having significant quantities of such acid gas components must be treated for removal of such impurities, or sweetened, before the natural gas therefrom can be marketed. Of necessity, the treatment to sweeten a natural gas produced from such a field adds to its cost of production. The character and content of the acid gas components in the natural gas of the field can approach levels where the added cost of treating the produced sour gas for removal of such impurities to produce a marketable sweet natural gas may make it uneconomical to exploit the gas field.
As noted, the processing of sour natural gas generally involves a gas sweetening process that splits the sour natural gas into two process streams: a sweet residue gas stream, and an acid gas stream. The sweet residue gas stream, typically made up of methane and light hydrocarbon ends, may go on to additional processing and hydrocarbon recovery or directly to gas sales. Impurities and sulphur compounds are removed from the acid gas stream leaving a carbon dioxide-rich acid gas stream. Often the carbon dioxide-rich product is needed at high pressure for carbon dioxide pipeline sales to enhanced oil recovery (EOR) projects or for reinjection into disposal wells. Alternatively, a high pressure carbon dioxide gas stream may be flashed to a low pressure and cold temperature, i.e., 90 psia, -60.degree. F., and used to provide process refrigeration.
In general, two basic approaches have been developed for use in processing natural gas containing significant quantities of carbon dioxide for the removal of that acid gas impurity. One approach employs physical solvents for the absorption and separation of the carbon dioxide impurity from the natural gas. In the solvent approach, the carbon dioxide containing natural gas is contacted countercurrently with a solvent in an absorption tower wherein the solvent physically absorbs carbon dioxide from the natural gas. Sweetened natural gas is recovered as an overhead gas stream from the absorption tower. The carbon dioxide-rich solvent is circulated to a regenerator vessel where the solvent is usually regenerated by pressure reduction, that is, when pressure on the carbon dioxide-rich solvent is reduced, carbon dioxide flashes out of the solvent as a gas. Hence, the solvent method produces a low pressure, vapor phase, carbon dioxide stream, and the regenerated lean solvent is recirculated to the absorption tower. At typical processing pressures, the required lean solvent circulation rate increases with increasing carbon dioxide content of the natural gas stream under treatment. As a result, higher solvent circulation rates are required to remove carbon dioxide from feed gases as the carbon dioxide concentration of the feed gas increases.
A second basic approach for removing carbon dioxide from a natural gas stream is cryogenic fractionation. In such process, methane and lighter components are separated from carbon dioxide and heavier components in a multi-stage tower or fractionator. Typically, the fractionator has both a stripping section and a rectifying section. In the stripping section, heat is supplied to the bottom of the tower to vaporize light hydrocarbon ends remaining in the liquified carbon dioxide bottoms, while in the rectifying section, condensed light hydrocarbons ends are recirculated to the top of the tower as a reflux to cool and condense carbon dioxide vapor remaining in the methane-rich vapor top ends. For such separation to be achieved, the fractionator must operate at cryogenic temperature, i.e., temperatures in the range of -60.degree. to -130.degree. F. As a result, high compressor horsepower is required to provide the external process refrigeration or, if product stream carbon dioxide is expanded for use as a refrigerant for operation of the cryogenic fractionator, to recompress the carbon dioxide.
Although both methodologies, physical solvents and cryogenic fractionation, can be applied to a high carbon dioxide content natural gas stream, the cost of utilizing either type of process for the treatment of a high carbon dioxide content natural gas stream, i.e., carbon dioxide greater than 40 mole percent, can be prohibitively expensive. In some special circumstances, cryogenic fractionation may be employed for separation of the CO.sub.2 content of a high carbon dioxide content methane gas stream to prepare a sales gas grade of methane. One such special circumstance is with regard to landfill gas as discussed in U.S. Pat. No. 4,681,612. From the standpoint of odors and safety, landfill gas must be removed from the landfill site in any event. Since the cost of producing landfill gas and the quantities of such gas to be processed are minuscule in comparison to that of production of natural gas from a natural gas field, employment of cryogenic fractionation to upgrade the methane content of such landfill gas to a marketable sales-gas quality may be justified on a cost basis.
However, the use of a physical solvent or a cryogenic fractionation process for the removal of carbon dioxide from a sour natural gas stream, by reason of the gas volumes involved, becomes exceedingly expensive when the carbon dioxide content is greater than about 40 mole %. The primary factors dictating the expense of treatment with regard to either process is due to the large equipment requirements caused by the high gas volumes to be processed. Further adding to the expense wherein cryogenic fractionation is utilized is the cost of special construction materials. Whereas normal carbon steel can be used at temperature of -20.degree. F. or greater, for operations at lower temperatures carbon steel must be specially treated. For operation at cryogenic temperatures, -50.degree. F. or less, special materials, such as nickel steels, are required. In a cryogenic fractionation process, the cost of providing external refrigeration and/or recompression costs for carbon dioxide if it is used as a refrigerant by expansion, becomes prohibitive. The special materials requirements taken together with the large equipment sizes required makes cryogenic fractionation economically unattractive. Likewise the physical solvent method is economically unattractive. For instance, in a physical solvent method that treats a natural gas stream containing 65 mole % carbon dioxide to produce 200 mscfd of market grade natural gas requires about 400 kbpd of solvent circulation to the absorber tower, and of course, the attendant cost of regeneration of such solvent. Further, the solvent method produces the carbon dioxide product stream as a gas stream. For the CO.sub.2 to be used for EOR application or to be reinjected into disposal wells, it must be recompressed which adds extra cost.
As a consequence, the development of a method which significantly reduces the cost of sweetening a high CO.sub.2 content natural gas would greatly add to the industry's motivation to develop and explore natural gas fields wherein the natural gas contains more than about 40 mole % carbon dioxide. Several such fields are known to exist having producible natural gas containing carbon dioxide concentrations of 65-70 mole % with some fields being as high as 80-90 mole % carbon dioxide.
For such high carbon dioxide content natural gas fields to be viable for exploitation, it is necessary to develop a more cost effective process for sweetening the natural gas from such fields to a marketable quality. It would be particularly desirable that such process be one which removes carbon dioxide as a high pressure moderately cold liquified product stream.